In solid-state physics, electron mobility characterizes how quickly an electron can move through a metal or semiconductor when pulled by an electric field. In semiconductors, there is an analogous quantity for holes, called hole mobility. When an electric field E is applied across a piece of material, the electrons respond by moving with an average velocity called the drift velocity (νd). Then the electron mobility μ is defined asνd=μE
When there is no applied electric field, electrons move around randomly in a solid, or in the case of semiconductors both electrons and holes move around randomly. Therefore, on average there will be no overall motion of charge carriers in any particular direction over time.
In the electronics industry there is a strong interest in materials known as superconductors or super semiconductors that demonstrate a very high mobility at room temperature. Super conductors have a band structure that is uniquely characterized by the symmetry near Fermi level [1]. The energy gap, Eg, in superconductors is very small (Eg<3.5 kTc, where k is Boltzman constant). Superconductivity in diamond heavily doped with boron was discovered in 1994. Covalent superconductivity in diamond was investigated and hole-pairing instead of electrons was used to explain these phenomena [2]. There are two general types of superconductors known at the present time, type I superconductors and type II superconductors.
Type I superconductors are basic elements or alloys that usually have a very low critical temperature (Tc). Below Tc, ohmic resistance drops to zero and the material can carry large DC current without any conductor loss. In addition to lossless DC conduction, a superconductor also emits a magnetic field below a critical value (Hc). Superconductivity of this type has been attributed to the formation of electron-pairs (Cooper pair) [1] which can flow effortlessly in the superconducting material. This is known as BCS Theory [1].
Type II superconductors are typically ceramic insulators at room temperature that exhibit superconducting characteristics below Tc. Tc in Type II superconductors is generally higher than those in Type I and often Type II is referred to as high temperature superconductors. Examples of this type of superconductor include YBaCuO, BiSrCaCuO, and HgBaCaCuO. There is no clear explanation on how Type II superconductors work. In general, this type of material consists of three components. The first component is CaO, SrO, BaO, or a mix of these oxides. This is the base material to form the superconducting crystal. The second component is CuO. The CuO is the acceptor material that generates holes near the Fermi level. The third component is Yo, Hg, BiO, PbO, or SnO, etc., which function as donor atoms to generate electrons near the Fermi level. The list of high temperature superconductors has been steadily growing since their initial discovery in 1986. The list now includes other elements than those mentioned above, including Fe and As, but not limited to those.
In 2008, scientists at the Brookhaven National Laboratory confirmed the possibility of electron pairing above the critical temperature of superconductors. This means, at least in theory, that a material can have very high mobility at room temperature regardless of whether the material is in a superconducting state or not. The key features of such a material are based on their band structure as: (1) the energy gap is near zero; and (2) the band structure is symmetric around the Fermi level to suggest pairing of electrons and/or holes.
To date, the only example of material that meets the criteria of a super semiconductor is Graphene. Graphene is a single layer of carbon atoms with a hexagonal crystal lattice. Because of its unique properties, Graphene recently has been the subject of extensive research. The measured mobility of doped Graphene is in the order of 10,000-20,000 cm2/Vs. The mobility of undoped Graphene is estimated at 200,000 cm2/Vs. This is more than 1000 times greater than bulk graphite, which has an electron mobility of 100 cm2/Vs. Graphene also has one of the longest known mean free paths to support ballistic electrons [4]. All of these characteristics suggest that Graphene meets the above criteria to be considered a super semiconductor.
However, graphene is difficult to grow in large wafers and the yield is low because of damage to the single layer material in the process of electronic device fabrication. Because of this, Graphene is used mainly for the fabrication of discrete devices.
GaN is gaining in popularity in high power electronic devices at microwave frequencies due to a high breakdown electric field, good thermal conductivity, and the ability to operate at high temperature without significant loss of performance. Also, the ability to grow GaN on silicon or SiC wafers promises low cost and a decent level of integration. The wide bandgap of GaN makes doping with other materials an effective tool to control the electrical properties of the lattice.
Most of the work on doping GaN focuses on acquiring conductivity. P or N type doping includes materials such as Mg and Si. Other materials such as copper (Cu) and arsenic (As) have been suggested to acquire other electrical properties. Doping GaN with copper produces magnetic properties that depend on the doping concentration [6, 7]. Doping GaN with arsenic [8] resulted in significantly shifting the emission spectrum of a GaN light emitting diode (LED) toward blue light and increasing the intensity of the emitted light by several orders of magnitude compared to undoped GaN LEDs.
Previous work on doping GaN with copper and arsenic used either copper or arsenic, but not both. Arsenic was doped into GaN at a very small percentage (<1%). Also arsenic was doped into both Ga and N sites. In previous work on doping with arsenic, the arsenic atoms mostly occupied the N sites that resulted in a downward shift in the spectrum [7].
It would be highly advantageous to enhance the mobility of electrons and holes in GaN without the deficiencies of the prior art.